Converting time
Time does not go in tens. A minute is 60 seconds, an hour is 60 minutes, and a day is 24 hours.
A worked example
Change 2.5 hours to minutes. Each hour is 60 minutes, so 2.5 × 60 = 150 minutes. Change 150 minutes to seconds: 150 × 60 = 9 000 seconds. Change 3 days to hours: 3 × 24 = 72 hours. Recap: multiply by 60 or 24 to go to a smaller unit, and divide to go to a bigger one.
The mistake people actually make
2.5 hours is 2 hours and 30 minutes, not 2 hours and 50 minutes. The digits after the decimal point are parts of 60, not parts of 100.